The invention relates to electrical machines comprising a rotary superconductive winding.
Electrical machines have been proposed which comprise a superconductive field winding disposed in a cryostat which can rotate around a shaft and is constructed to be supplied with direct current to produce a field rotating at the same speed as the field winding; stator windings adapted to be flowed through by three-phase currents to produce a rotary field cooperating with the field winding; and electrically conductive rotary means disposed between the field winding and the stator windings and adapted to be flowed through, during operation, by currents producing a field which tends to counteract the torque exerted on the field winding. The superconductive field winding rotates at the same speed as the rotary field supplied by the stator winding. Consequently, it is subjected only to a constant or very slowly-varying magnetic field, thus obviating the results of the magnetic hysteresis of superconductive materials. Furthermore, since no appreciable driving force is transmitted between the superconductive field winding and the components at room temperature, the necessary connections can have a sufficiently high thermal resistance to reduce to an acceptable value the rate of cooling required for maintaining the winding at superconductivity temperatures.
The present invention particularly relates to synchronous rotary machines, this term being used to define a machine wherein the electrically conductive means produce a field rotating in sychronism with the field winding. Machines of this kind are described in copending U.S. patent application Ser. No. 443,014 assigned to the assignee of the present invention. However, the invention also applies to asynchronous machines as described in copending U.S. patent application Ser. No. 412,127, now U.S. Pat. No. 3,904,901 and in U.S. Pat. No. 3,742,265 (Smith). The invention also applies to rotary transformers wherein one of the primary and secondary windings comprises stator windings and the other of which comprises windings comprising the aforementioned electrically conductive rotary means.
The units given in the aforementioned patent applications may be conisdered "cylindrical" in that the lines of force of the magnetic fields have a substantially radial orientation between the different field-producing means. That arrangement is satisfactory when the machine has a small number of poles (particularly in a two-pole machine) but has substantial drawbacks in the case of motors having a large number of poles. On the other hand, in some applications a large number of poles are required so that the shaft to which mechanical power is applied or which delivers power rotates at a low speed for a currently available a.c. frequency. This problem occurs inter alia in naval propulsion: propulsion assemblies are known which comprise a turbo-alternator rotating at high speed and supplying an asynchronous electric motor which is mounted directly on the propeller shaft and which must therefore be considerably slower than the turbo-alternator. To this end, the asychronous motor windings must have a large number of poles. If the number of poles is large, the field winding must have a large diameter and its flux is used on one side of the winding only (i.e. inefficiently) and the winding is relatively short relative to its diameter, resulting in a relatively high proportion of current return conductors, which play only a secondary part in the production of flux.